# Predicting The Crash Site For The Tiangong-1

Astronomers have been able to calculate accurately the movement of certain space objects (planets, stars), to tell when and where they will be with high precision. It is interesting that close to the Earth, the exact impact of the Tiangong-1 space station it is hard to calculate. In recent months, astronomers have made calculations, predictions and followed the Tiangong-1 orbit.

Let us see how it started:

• China launched the Shenzhou-1 spacecraft in 1999 and in 2003 the Shenzhou-5 spacecraft aboard Yang Liwei taikonaut, making China the third country in the world to become a successful space program.
• In 1992, China began building its first space station program – the Tiangong program, independent of other international countries.
• China launched Tiangong-1 space station, its first space laboratory (target vehicle, docking technology) on September 29, 2011; Tiangong-2 the second advanced space laboratory was launched on September 15, 2016. In future, the third space station Tiangong-3 will continue to enlarge these new technologies.

Satellite Tiangong-1 is as big as a school bus (8506 kg) it is 10.4 m long and has a maximum diameter of 3.4 m. It has two modules: a resource and an experiment module with an effective volume of 15 m3.

After the Chinese space project, the Tiangong-1 space station (see Fig. 1) would have to return to earth in 2013, but it remained active until 2016. Amateur astronomers signaled in 2016 that Tiangong-1 was not under control. In September 2016, the Chinese admitted that they had lost contact with it and they predicted Tiangong-1 would fall to Earth in 2018. In January 2018 after Reuters, the Chinese said that they have control over it.

Tiangong-1 (NORAD 37820) orbits from west to east, taking about 4 minutes to cross the sky. From Romania, we will see the motion of Tiangong-1 satellite at low altitude at evening in the beginning of March (magnitude 1.6). The observation of Tiangong-1 will be interesting in the next period, especially the re-entry, if we can catch the moment.

We studied the motion of Tiangong-1 satellite numerically using the simplest equations of motion under the action of atmospheric drag and effect of oblateness of Earth.

The motion of the satellite under the actions of various forces is fitting of its trajectory. The most important perturbing force is gravitation, the attraction of Earth. Other perturbations on an artificial satellite can be the effect of the solar and lunar attraction, the effect of the solar radiation pressure, the effect of the Earth’s magnetic field, the effect of the electrostatic field existing in the ionosphere, the effect of the radiation reflected from the Earth, the collisions with micrometeorites, the relativity effect, etc.

Fig. 2 – Modelled Tiangong-1 orbit before the re-entry. Credit: Iharka Szücs-Csillik

The presented simplified numerical method is very useful to understand how orbit perturbations realized. Numerical results have depicted the simulation of satellite motion until satellite re-entry (Fig. 2). Low Earth orbiting satellites encounter orbital decay, their interaction with the atmosphere determine their physical lifetimes. Therefore, predicting the Tiangong-1 space station reentry’s timing and the location is difficult. Prediction of such lifetimes or a re-entry date is of great interest to satellite observers.

In Fig. 3, one can see the variation of radius distance in megameter versus time (minutes). The perigee distance decreases from roughly 260 km in middle February at circa 245 km at the end of February. Investigating other satellite’s reentries reveal that the perigee altitude target at re-entry is approximately 100-150 km. When a satellite reaches this target altitude, then after 600-1000 seconds it will collide with Earth. The moment depends on local circumstances (atmosphere, elevation of terrain, etc.) too.

Fig. 3 Tiangong-1 re-entry (radius vector (Mm) versus time (min)). Credit: Iharka Szücs-Csillik

The Tiangong-1 space station (space debris) will re-enter somewhere between 43o North and 43o South latitude (see here). Using two-line element data (provide from Dr. T.S. Kelso), and our numerical integrator we can affirm that the Tiangong-1 re-entry can occur around late March or at the beginning of April in 2018.

This research was described in the article entitled ANALYSIS AND PREDICTION OF TIANGONG-1 REENTRY, published in the Romanian Astronomical Journal. This work was led by Iharka Szücs-Csillik from the Astronomical Institute of Romanian Academy, Cluj-Napoca.

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